FALMOUTH FIELDCOURSE

Group 5

The views expressed are those of the individuals concerned and do not express the views of the University of Southampton or those of the National Oceanography Centre Southampton.

Minibat data

The temperature is highest at the surface reaching a maximum of 15.5°C. It then decreases at higher pressures. Temperature decreases in a fairly uniform fashion because the higher the pressure, the deeper the water and therefore the less sunlight can reach, warming the waters.  

Temperature

Salinity

Salinity increases as pressure increases. This is because the more saline the water, the denser it is causing the water to sink. There is an area of higher salinity at the surface, this may be due to the evaporation of surface waters, reducing the freshwater therefore increasing the salinity.

Density

Density increases as pressure increases. This is because the denser water will sink causing the less dense water to be present at the surface. The density of the water is affected by its temperature and salinity, therefore, the areas with higher density at the surface could be areas of colder and more saline water.

Fluorescence

Fluorescence appears to occur between pressures of 15 – 22 db. There are clusters of high fluorescence occurring at irregular intervals caused by a group of plankton. The area where the fluorescence reach higher pressures (26 db) is caused by plankton being present at a lower depth, this may be due to a larger amount of mixing occurring here.

Turbidity

Turbidity is generally low as the wind speed was down and the weather was low causing the sea to be calm. There are small spots of high turbidity levels occurring at around 18 db, this could potentially be caused by the plankton present at the deep chlorophyll maximum.  

The minibat was deployed when the final station (C8) had been sampled on the return journey to Falmouth. It was deployed from the deck of the Callista with the wings at maximum elevation to keep it as shallow as possible before data started being logged. Data started being recorded when the fill 100m of cable had been deployed with the limits of the minibat initially set to between 10 and 29m meaning the minibat was set to automatically change its wing angle to rise when it reached the lower limit or dive at the upper limit. The minibat failed to reach the upper limit and so the speed was increased from 3kn to 4kn to aid in ascent. At this speed the minibat was however not reaching a depth of 29m when the wings were at their maximum negative elevation and so the bottom limit was set to 28m which it could reach.

The minibat was in the water from 14:23 to 15:53 UTC during which the Callista travelled a distance of 5.5 nautical miles. Location was taken at 10 minute intervals to accurately plot the path taken which passed the previous station (C7) before the minibat was recovered. The track was monitored whilst the minibat was in the water to ensure it hadn’t flipped or was having any other problems that could affect the data collected or its ability to climb or dive.

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Richardson Number data

To establish the stability of the water column, the Richardson’s Number (Ri) can be obtained from ADCP and CTD data. Each graph shows an overlay of a temperature profile from the CTD and how the Ri changes with depth.

The Richardson’s number is an equation used in stable, stratified water columns to determine where there are areas of turbulence (Galperin et.al, 2007). A Richardson’s number of below 0.25 shows a layer that is likely to have mechanical mixing occurring – it will have an almost uniform density throughout and will display turbulent flow. Conversely, an Ri of above 1 shows a stable layer with laminar flow, unlikely to mix vertically through the column meaning a density gradient can be observed. Between these two critical values is the ‘Grey area’ in which the column transitions between stable and unstable, and flow type is dependent on the surrounding column stability.

Station C4 shows a very weak thermocline, and therefore low levels of stratification and a generally well mixed column, with only one Ri above 1 to indicate a mixed surface layer around 11m deep. Station C5 shows stronger stratification, with a 3oC change between 10 and 20m, and definite density interfaces shown by 4 Ri values above 1 and a number below 1 in the ‘Grey area’. C6 again shows a steady change in density and temperature from the surface to 20m, similar to C5, supported by stratification indicators from the Ri values showing density interfaces at 10 and 20m. Stations C7 and C8 show strong thermoclines at 20m, indicating stratification, with obvious corresponding Ri values at the respective depths. The depth of the density interfaces, and therefore mixed surface layer (euphotic zone), can be seen to slightly decrease throughout the day (C4 to C8) as temperature increases stratification.

References:

Galperin, B., Sukoriansky, S. and Anderson, P. (2007). On the critical Richardson number in stably stratified turbulence. Atmospheric Science Letters, 8(3), pp.65-69.

CTD data

Station C4 was the ‘shakedown’ station, it allowed us to practise the methods we would be using at further stations. It was relatively close to shore and was therefore likely to be the most well mixed station we measured.

This is reflected in the data shown in the graph. Temperature, shown in red, decreases rapidly in the first few meters before becoming more gradual with depth. The salinity, shown in blue, is relatively constant throughout the water column. There is high variability in the surface metre of the water column, this is likely to be the result of local weather change, i.e. rainfall and evapouration. The fluorescence increases with depth to ~15m where is begins to decrease. This suggests that phytoplankton are residing in the middle of the water column. There is no obvious deep chlorophyll maximum, thus indicating a lack of stratification at this site.

Station C4

Station C5 was the repeat station at which Terramare remained. This station was further out to sea and was therefore likely to be less well mixed.

The temperature measurements show a strong thermocline between ~10 and 20m. The fluorescence shows a large peak at ~20m, this is the deep chlororphyll maximum (DCM). The DCM and base of the thermocline both sit at ~20m, suggesting that the water column is stratified. The salinity varies throughout that water column, it becomes less variable below 20m suggesting that the surface waters experience more mixing.

Station C5

Station C6 shows high stratification. There is a thermocline evident from ~5-20m. This again coincides with fluorescence peak showing the presence of the DCM. Again, salinity was variable but became more homogenous with depth.

Station C6

Station C7 shows strong stratification. The thermocline shown between 5 and 20m is relatively steep and at its base lies the deep chlorophyll maximum. The salinity is variable above 20m but below this point it becomes homogenous, again suggesting that there is a large amount of mixing in the upper water column.

Station C7

The CTD data from station C8 shows a strong thermocline from 1-20m. The salinity is variable above 20m, after which it becomes more homogenous. The fluorescence shows a strong peak at the base of the thermocline, this is the deep chlorophyll maximum.

The deep chlorophyll maximum reflects stratification as it shows the point at which phytoplankton are most productive. Phytoplankton require light and heat to photosynthesise, they also need sufficient nutrients. In stratified waters, the strong thermocline prevents mixing between the nutrient rich deep water and the nutrient depleted surface water; this explains why the DCM lies at the boundary between the two.

Station C8

Click below to see our physical data from the estuary and the pontoon at King Harry Ferry

Click below for more data from offshore

Fluorescence and Turbidity

The fluorescence at station C4 increases with depth to a peak at ~15m, here it begins to decrease as depth increases. There is a second peak at ~20m depth. The turbidity varies throughout the water column but remains within the range 0.22 – 0.27, towards the seafloor the turbidity begins to increase. This is to be expected as sediment disturbed sediments becomes suspended in the water column.

Station C4

The fluorescence in this case shows a large peak at ~20m depth, this is the deep chlorophyll maximum as identified in the CTD data. The turbidity shows two peaks. The first at 20m, this is the result of high concentrations of phytoplankton and zooplankton in the water column. The stronger the DCM, the more cells present and thus the better the feeding opportunity for zooplankton, increasing their numbers. The second peak is at the seafloor, this reflects suspended sediment from the seabed.

Station C5

Both the fluorescence and the turbidity for station C6 show a very similar pattern. The fluorescence peak at ~15-20m reflects the deep chlorophyll maximum. The turbidity peak mirroring the DCM can be explained by high number of planktonic cells in the water column. The large phytoplankton assemblage attracts a large number of zooplankton further increasing the turbidity.

Station C6

The fluorescence peaks just above 20m, this is to be expected and reflects the deep chlorophyll maximum. The turbidity shows a similar pattern to the fluorescence. The peak just above 20m is likely explained by the high planktonic cell count. The narrow peak at the base of water column can be explained by an increase in suspended sediment near the benthos.

Station C7

Station C8

The fluorescence at this station shows a strong deep chlorophyll maximum at ~20m. The turbidity at this station has peak at the surface, this is likely to be caused by bubbles from a boat’s wake or by wave action. There is a small peak at the DCM, this is smaller than at previous stations suggesting that there are fewer plankton cells present. The slight increase at the base of the water column is likely to be caused by sediment suspension.

Light data

The CTD on Callista provided its own PAR data, which was plotted against depth, rather than needing to use a Transmissometer (See graphs to the right). This also means surface Irradiance can only be estimated from the shallowest depth, which proved to be inaccurate compared to the Transmissometer. The depth of the 1% light level can be found at each station (C4 through C8) by finding the depth at which the light level corresponds with the lowest known amount of Irradiance phytoplankton require to photosynthesise, before light becomes a limiting factor.

Click each graph to expand

Using an average of 3molPhotons/m2/per day (Venables & Moore, 2010), the depths of the Euphotic can be estimated as seen in the table to the right.

The base of the euphotic zone, estimated from the 1% light depth, can be seen to match the depth of the DCM at station C5, C7 and C8 from our measured chlorophyll concentrations (C4 and C6 had inconclusive data). This suggests Callista’s 1% light data estimate is accurate.

Click here for Callista’s chlorophyll data